1. Field of the Invention
The invention relates to a device for compensating the temperature drifts of the gain of an ultra-high frequency electric signal amplifier.
2. Description of the Prior Art
It is known to stabilize the gain of an amplifier by means of a regulation loop. According to this process, a part of the output power of the amplifier is taken off by means of a coupler and is applied to the input of a detector, formed generally by a detection diode whose drift is temperature compensated and which delivers a DC voltage to a first input of a comparator. A threshold or reference DC voltage is applied to the second input of the comparator. The output of the comparator delivers an error signal whose amplitude is proportional to the difference between the detected voltage and the threshold or reference voltage. The error signal obtained acts on the attenuation ratio of an attenuator placed in the amplification chain, so that the power level at the output of the amplifier is held constant whatever the variations of the temperature.
A problem however arises when the amplifier transmits simultaneously several frequencies with variable frequency differences, for in this case the voltage at the output of the detector no longer varies solely as a function of the temperatures variations but also as a function of the different transmitted frequencies, and accordingly it is no longer possible to compensate directly the variations in gain due to the temperature by the voltage delivered by the detector.
It is also known to stabilize the gain of an amplifier by acting directly on the biasing values of the amplifier transistors. This process consists, for example, in compensating the temperature drift of a field effect transistor by applying a correction voltage to the control gate of the transistor, which causes a variation of the drain current and thus a gain variation. To implement this process, it is sufficient to choose a temperature responsive element which acts correctly on the biasing voltage of the gate of the transistor. However, this process which is applicable to low level transistors, working far from their saturation zones, is very difficult to put into practice with power field effect transistors. In fact, experience shows that the average drift of an amplifying chain of field effect transistors is substantially 0.015 decibels per degree centigrade multiplied by the number of stages forming the amplifier, which leads for example, for an amplifying chain with 8 cascade connected transistors subjected to temperature variations within a range for example of 100.degree., to a peak to peak drift of practically 12 decibels. Because of the great correction dynamics required, the processes for compensating by acting on the biasing of the transistors thus appear totally inefficient.